<!doctype html><html lang="en">
 <head>
  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">
  <title>Web Background Synchronization</title>
  <meta content="width=device-width, initial-scale=1, shrink-to-fit=no" name="viewport">
<style data-fill-with="stylesheet">/******************************************************************************
 *                   Style sheet for the W3C specifications                   *
 *
 * Special classes handled by this style sheet include:
 *
 * Indices
 *   - .toc for the Table of Contents (<ol class="toc">)
 *     + <span class="secno"> for the section numbers
 *   - #toc for the Table of Contents (<nav id="toc">)
 *   - ul.index for Indices (<a href="#ref">term</a><span>, in §N.M</span>)
 *   - table.index for Index Tables (e.g. for properties or elements)
 *
 * Structural Markup
 *   - table.data for general data tables
 *     -> use 'scope' attribute, <colgroup>, <thead>, and <tbody> for best results !
 *     -> use <table class='complex data'> for extra-complex tables
 *     -> use <td class='long'> for paragraph-length cell content
 *     -> use <td class='pre'> when manual line breaks/indentation would help readability
 *   - dl.switch for switch statements
 *   - ol.algorithm for algorithms (helps to visualize nesting)
 *   - .figure and .caption (HTML4) and figure and figcaption (HTML5)
 *     -> .sidefigure for right-floated figures
 *   - ins/del
 *
 * Code
 *   - pre and code
 *
 * Special Sections
 *   - .note       for informative notes             (div, p, span, aside, details)
 *   - .example    for informative examples          (div, p, pre, span)
 *   - .issue      for issues                        (div, p, span)
 *   - .assertion  for assertions                    (div, p, span)
 *   - .advisement for loud normative statements     (div, p, strong)
 *   - .annoying-warning for spec obsoletion notices (div, aside, details)
 *
 * Definition Boxes
 *   - pre.def   for WebIDL definitions
 *   - table.def for tables that define other entities (e.g. CSS properties)
 *   - dl.def    for definition lists that define other entitles (e.g. HTML elements)
 *
 * Numbering
 *   - .secno for section numbers in .toc and headings (<span class='secno'>3.2</span>)
 *   - .marker for source-inserted example/figure/issue numbers (<span class='marker'>Issue 4</span>)
 *   - ::before styled for CSS-generated issue/example/figure numbers:
 *     -> Documents wishing to use this only need to add
 *        figcaption::before,
 *        .caption::before { content: "Figure "  counter(figure) " ";  }
 *        .example::before { content: "Example " counter(example) " "; }
 *        .issue::before   { content: "Issue "   counter(issue) " ";   }
 *
 * Header Stuff (ignore, just don't conflict with these classes)
 *   - .head for the header
 *   - .copyright for the copyright
 *
 * Miscellaneous
 *   - .overlarge for things that should be as wide as possible, even if
 *     that overflows the body text area. This can be used on an item or
 *     on its container, depending on the effect desired.
 *     Note that this styling basically doesn't help at all when printing,
 *     since A4 paper isn't much wider than the max-width here.
 *     It's better to design things to fit into a narrower measure if possible.
 *   - js-added ToC jump links (see fixup.js)
 *
 ******************************************************************************/

/******************************************************************************/
/*                                   Body                                     */
/******************************************************************************/

	body {
		counter-reset: example figure issue;

		/* Layout */
		max-width: 50em;               /* limit line length to 50em for readability   */
		margin: 0 auto;                /* center text within page                     */
		padding: 1.6em 1.5em 2em 50px; /* assume 16px font size for downlevel clients */
		padding: 1.6em 1.5em 2em calc(26px + 1.5em); /* leave space for status flag     */

		/* Typography */
		line-height: 1.5;
		font-family: sans-serif;
		widows: 2;
		orphans: 2;
		word-wrap: break-word;
		overflow-wrap: break-word;
		hyphens: auto;

		/* Colors */
		color: black;
		background: white top left fixed no-repeat;
		background-size: 25px auto;
	}


/******************************************************************************/
/*                         Front Matter & Navigation                          */
/******************************************************************************/

/** Header ********************************************************************/

	div.head { margin-bottom: 1em }
	div.head hr { border-style: solid; }

	div.head h1 {
		font-weight: bold;
		margin: 0 0 .1em;
		font-size: 220%;
	}

	div.head h2 { margin-bottom: 1.5em;}

/** W3C Logo ******************************************************************/

	.head .logo {
		float: right;
		margin: 0.4rem 0 0.2rem .4rem;
	}

	.head img[src*="logos/W3C"] {
		display: block;
		border: solid #1a5e9a;
		border-width: .65rem .7rem .6rem;
		border-radius: .4rem;
		background: #1a5e9a;
		color: white;
		font-weight: bold;
	}

	.head a:hover > img[src*="logos/W3C"],
	.head a:focus > img[src*="logos/W3C"] {
		opacity: .8;
	}

	.head a:active > img[src*="logos/W3C"] {
		background: #c00;
		border-color: #c00;
	}

	/* see also additional rules in Link Styling section */

/** Copyright *****************************************************************/

	p.copyright,
	p.copyright small { font-size: small }

/** Back to Top / ToC Toggle **************************************************/

	@media print {
		#toc-nav {
			display: none;
		}
	}
	@media not print {
		#toc-nav {
			position: fixed;
			z-index: 2;
			bottom: 0; left: 0;
			margin: 0;
			min-width: 1.33em;
			border-top-right-radius: 2rem;
			box-shadow: 0 0 2px;
			font-size: 1.5em;
			color: black;
		}
		#toc-nav > a {
			display: block;
			white-space: nowrap;

			height: 1.33em;
			padding: .1em 0.3em;
			margin: 0;

			background: white;
			box-shadow: 0 0 2px;
			border: none;
			border-top-right-radius: 1.33em;
			background: white;
		}
		#toc-nav > #toc-jump {
			padding-bottom: 2em;
			margin-bottom: -1.9em;
		}

		#toc-nav > a:hover,
		#toc-nav > a:focus {
			background: #f8f8f8;
		}
		#toc-nav > a:not(:hover):not(:focus) {
			color: #707070;
		}

		/* statusbar gets in the way on keyboard focus; remove once browsers fix */
		#toc-nav > a[href="#toc"]:not(:hover):focus:last-child {
			padding-bottom: 1.5rem;
		}

		#toc-nav:not(:hover) > a:not(:focus) > span + span {
			/* Ideally this uses :focus-within on #toc-nav */
			display: none;
		}
		#toc-nav > a > span + span {
			padding-right: 0.2em;
		}

		#toc-toggle-inline {
			vertical-align: 0.05em;
			font-size: 80%;
			color: gray;
			color: hsla(203,20%,40%,.7);
			border-style: none;
			background: transparent;
			position: relative;
		}
		#toc-toggle-inline:hover:not(:active),
		#toc-toggle-inline:focus:not(:active) {
			text-shadow: 1px 1px silver;
			top: -1px;
			left: -1px;
		}

		#toc-nav :active {
			color: #C00;
		}
	}

/** ToC Sidebar ***************************************************************/

	/* Floating sidebar */
	@media screen {
		body.toc-sidebar #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			max-width: 80%;
			max-width: calc(100% - 2em - 26px);
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body.toc-sidebar #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}
		body.toc-sidebar #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	/* Hide main scroller when only the ToC is visible anyway */
	@media screen and (max-width: 28em) {
		body.toc-sidebar {
			overflow: hidden;
		}
	}

	/* Sidebar with its own space */
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) #toc {
			position: fixed;
			top: 0; bottom: 0;
			left: 0;
			width: 23.5em;
			overflow: auto;
			padding: 0 1em;
			padding-left: 42px;
			padding-left: calc(1em + 26px);
			background: inherit;
			background-color: #f7f8f9;
			z-index: 1;
			box-shadow: -.1em 0 .25em rgba(0,0,0,.1) inset;
		}
		body:not(.toc-inline) #toc h2 {
			margin-top: .8rem;
			font-variant: small-caps;
			font-variant: all-small-caps;
			text-transform: lowercase;
			font-weight: bold;
			color: gray;
			color: hsla(203,20%,40%,.7);
		}

		body:not(.toc-inline) {
			padding-left: 29em;
		}
		/* See also Overflow section at the bottom */

		body:not(.toc-inline) #toc-jump:not(:focus) {
			width: 0;
			height: 0;
			padding: 0;
			position: absolute;
			overflow: hidden;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) {
			margin: 0 4em;
		}
	}

/******************************************************************************/
/*                                Sectioning                                  */
/******************************************************************************/

/** Headings ******************************************************************/

	h1, h2, h3, h4, h5, h6, dt {
		page-break-after: avoid;
		page-break-inside: avoid;
		font: 100% sans-serif;   /* Reset all font styling to clear out UA styles */
		font-family: inherit;    /* Inherit the font family. */
		line-height: 1.2;        /* Keep wrapped headings compact */
		hyphens: manual;         /* Hyphenated headings look weird */
	}

	h2, h3, h4, h5, h6 {
		margin-top: 3rem;
	}

	h1, h2, h3 {
		color: #005A9C;
		background: transparent;
	}

	h1 { font-size: 170%; }
	h2 { font-size: 140%; }
	h3 { font-size: 120%; }
	h4 { font-weight: bold; }
	h5 { font-style: italic; }
	h6 { font-variant: small-caps; }
	dt { font-weight: bold; }

/** Subheadings ***************************************************************/

	h1 + h2,
	#subtitle {
		/* #subtitle is a subtitle in an H2 under the H1 */
		margin-top: 0;
	}
	h2 + h3,
	h3 + h4,
	h4 + h5,
	h5 + h6 {
		margin-top: 1.2em; /* = 1 x line-height */
	}

/** Section divider ***********************************************************/

	:not(.head) > hr {
		font-size: 1.5em;
		text-align: center;
		margin: 1em auto;
		height: auto;
		border: transparent solid 0;
		background: transparent;
	}
	:not(.head) > hr::before {
		content: "\2727\2003\2003\2727\2003\2003\2727";
	}

/******************************************************************************/
/*                            Paragraphs and Lists                            */
/******************************************************************************/

	p {
		margin: 1em 0;
	}

	dd > p:first-child,
	li > p:first-child {
		margin-top: 0;
	}

	ul, ol {
		margin-left: 0;
		padding-left: 2em;
	}

	li {
		margin: 0.25em 0 0.5em;
		padding: 0;
	}

	dl dd {
		margin: 0 0 .5em 2em;
	}

	.head dd + dd { /* compact for header */
		margin-top: -.5em;
	}

	/* Style for algorithms */
	ol.algorithm ol:not(.algorithm),
	.algorithm > ol ol:not(.algorithm) {
	 border-left: 0.5em solid #DEF;
	}

	/* Put nice boxes around each algorithm. */
	[data-algorithm]:not(.heading) {
	  padding: .5em;
	  border: thin solid #ddd; border-radius: .5em;
	  margin: .5em calc(-0.5em - 1px);
	}
	[data-algorithm]:not(.heading) > :first-child {
	  margin-top: 0;
	}
	[data-algorithm]:not(.heading) > :last-child {
	  margin-bottom: 0;
	}

	/* Style for switch/case <dl>s */
	dl.switch > dd > ol.only,
	dl.switch > dd > .only > ol {
	 margin-left: 0;
	}
	dl.switch > dd > ol.algorithm,
	dl.switch > dd > .algorithm > ol {
	 margin-left: -2em;
	}
	dl.switch {
	 padding-left: 2em;
	}
	dl.switch > dt {
	 text-indent: -1.5em;
	 margin-top: 1em;
	}
	dl.switch > dt + dt {
	 margin-top: 0;
	}
	dl.switch > dt::before {
	 content: '\21AA';
	 padding: 0 0.5em 0 0;
	 display: inline-block;
	 width: 1em;
	 text-align: right;
	 line-height: 0.5em;
	}

/** Terminology Markup ********************************************************/


/******************************************************************************/
/*                                 Inline Markup                              */
/******************************************************************************/

/** Terminology Markup ********************************************************/
	dfn   { /* Defining instance */
		font-weight: bolder;
	}
	a > i { /* Instance of term */
		font-style: normal;
	}
	dt dfn code, code.idl {
		font-size: medium;
	}
	dfn var {
		font-style: normal;
	}

/** Change Marking ************************************************************/

	del { color: red;  text-decoration: line-through; }
	ins { color: #080; text-decoration: underline;    }

/** Miscellaneous improvements to inline formatting ***************************/

	sup {
		vertical-align: super;
		font-size: 80%
	}

/******************************************************************************/
/*                                    Code                                    */
/******************************************************************************/

/** General monospace/pre rules ***********************************************/

	pre, code, samp {
		font-family: Menlo, Consolas, "DejaVu Sans Mono", Monaco, monospace;
		font-size: .9em;
		page-break-inside: avoid;
		hyphens: none;
		text-transform: none;
	}
	pre code,
	code code {
		font-size: 100%;
	}

	pre {
		margin-top: 1em;
		margin-bottom: 1em;
		overflow: auto;
	}

/** Inline Code fragments *****************************************************/

  /* Do something nice. */

/******************************************************************************/
/*                                    Links                                   */
/******************************************************************************/

/** General Hyperlinks ********************************************************/

	/* We hyperlink a lot, so make it less intrusive */
	a[href] {
		color: #034575;
		text-decoration: none;
		border-bottom: 1px solid #707070;
		/* Need a bit of extending for it to look okay */
		padding: 0 1px 0;
		margin: 0 -1px 0;
	}
	a:visited {
		border-bottom-color: #BBB;
	}

	/* Use distinguishing colors when user is interacting with the link */
	a[href]:focus,
	a[href]:hover {
		background: #f8f8f8;
		background: rgba(75%, 75%, 75%, .25);
		border-bottom-width: 3px;
		margin-bottom: -2px;
	}
	a[href]:active {
		color: #C00;
		border-color: #C00;
	}

	/* Backout above styling for W3C logo */
	.head .logo,
	.head .logo a {
		border: none;
		text-decoration: none;
		background: transparent;
	}

/******************************************************************************/
/*                                    Images                                  */
/******************************************************************************/

	img {
		border-style: none;
	}

	/* For autogen numbers, add
	   .caption::before, figcaption::before { content: "Figure " counter(figure) ". "; }
	*/

	figure, .figure, .sidefigure {
		page-break-inside: avoid;
		text-align: center;
		margin: 2.5em 0;
	}
	.figure img,    .sidefigure img,    figure img,
	.figure object, .sidefigure object, figure object {
		max-width: 100%;
		margin: auto;
	}
	.figure pre, .sidefigure pre, figure pre {
		text-align: left;
		display: table;
		margin: 1em auto;
	}
	.figure table, figure table {
		margin: auto;
	}
	@media screen and (min-width: 20em) {
		.sidefigure {
			float: right;
			width: 50%;
			margin: 0 0 0.5em 0.5em
		}
	}
	.caption, figcaption, caption {
		font-style: italic;
		font-size: 90%;
	}
	.caption::before, figcaption::before, figcaption > .marker {
		font-weight: bold;
	}
	.caption, figcaption {
		counter-increment: figure;
	}

	/* DL list is indented 2em, but figure inside it is not */
	dd > .figure, dd > figure { margin-left: -2em }

/******************************************************************************/
/*                             Colored Boxes                                  */
/******************************************************************************/

	.issue, .note, .example, .assertion, .advisement, blockquote {
		padding: .5em;
		border: .5em;
		border-left-style: solid;
		page-break-inside: avoid;
	}
	span.issue, span.note {
		padding: .1em .5em .15em;
		border-right-style: solid;
	}

	.issue,
	.note,
	.example,
	.advisement,
	.assertion,
	blockquote {
		margin: 1em auto;
	}
	.note  > p:first-child,
	.issue > p:first-child,
	blockquote > :first-child {
		margin-top: 0;
	}
	blockquote > :last-child {
		margin-bottom: 0;
	}

/** Blockquotes ***************************************************************/

	blockquote {
		border-color: silver;
	}

/** Open issue ****************************************************************/

	.issue {
		border-color: #E05252;
		background: #FBE9E9;
		counter-increment: issue;
		overflow: auto;
	}
	.issue::before, .issue > .marker {
		text-transform: uppercase;
		color: #AE1E1E;
		padding-right: 1em;
		text-transform: uppercase;
	}
	/* Add .issue::before { content: "Issue " counter(issue) " "; } for autogen numbers,
	   or use class="marker" to mark up the issue number in source. */

/** Example *******************************************************************/

	.example {
		border-color: #E0CB52;
		background: #FCFAEE;
		counter-increment: example;
		overflow: auto;
		clear: both;
	}
	.example::before, .example > .marker {
		text-transform: uppercase;
		color: #827017;
		min-width: 7.5em;
		display: block;
	}
	/* Add .example::before { content: "Example " counter(example) " "; } for autogen numbers,
	   or use class="marker" to mark up the example number in source. */

/** Non-normative Note ********************************************************/

	.note {
		border-color: #52E052;
		background: #E9FBE9;
		overflow: auto;
	}

	.note::before, .note > .marker,
	details.note > summary::before,
	details.note > summary > .marker {
		text-transform: uppercase;
		display: block;
		color: hsl(120, 70%, 30%);
	}
	/* Add .note::before { content: "Note"; } for autogen label,
	   or use class="marker" to mark up the label in source. */

	details.note > summary {
		display: block;
		color: hsl(120, 70%, 30%);
	}
	details.note[open] > summary {
		border-bottom: 1px silver solid;
	}

/** Assertion Box *************************************************************/
	/*  for assertions in algorithms */

	.assertion {
		border-color: #AAA;
		background: #EEE;
	}

/** Advisement Box ************************************************************/
	/*  for attention-grabbing normative statements */

	.advisement {
		border-color: orange;
		border-style: none solid;
		background: #FFEECC;
	}
	strong.advisement {
		display: block;
		text-align: center;
	}
	.advisement > .marker {
		color: #B35F00;
	}

/** Spec Obsoletion Notice ****************************************************/
	/* obnoxious obsoletion notice for older/abandoned specs. */

	details {
		display: block;
	}
	summary {
		font-weight: bolder;
	}

	.annoying-warning:not(details),
	details.annoying-warning:not([open]) > summary,
	details.annoying-warning[open] {
		background: #fdd;
		color: red;
		font-weight: bold;
		padding: .75em 1em;
		border: thick red;
		border-style: solid;
		border-radius: 1em;
	}
	.annoying-warning :last-child {
		margin-bottom: 0;
	}

@media not print {
	details.annoying-warning[open] {
		position: fixed;
		left: 1em;
		right: 1em;
		bottom: 1em;
		z-index: 1000;
	}
}

	details.annoying-warning:not([open]) > summary {
		text-align: center;
	}

/** Entity Definition Boxes ***************************************************/

	.def {
		padding: .5em 1em;
		background: #DEF;
		margin: 1.2em 0;
		border-left: 0.5em solid #8CCBF2;
	}

/******************************************************************************/
/*                                    Tables                                  */
/******************************************************************************/

	th, td {
		text-align: left;
		text-align: start;
	}

/** Property/Descriptor Definition Tables *************************************/

	table.def {
		/* inherits .def box styling, see above */
		width: 100%;
		border-spacing: 0;
	}

	table.def td,
	table.def th {
		padding: 0.5em;
		vertical-align: baseline;
		border-bottom: 1px solid #bbd7e9;
	}

	table.def > tbody > tr:last-child th,
	table.def > tbody > tr:last-child td {
		border-bottom: 0;
	}

	table.def th {
		font-style: italic;
		font-weight: normal;
		padding-left: 1em;
		width: 3em;
	}

	/* For when values are extra-complex and need formatting for readability */
	table td.pre {
		white-space: pre-wrap;
	}

	/* A footnote at the bottom of a def table */
	table.def           td.footnote {
		padding-top: 0.6em;
	}
	table.def           td.footnote::before {
		content: " ";
		display: block;
		height: 0.6em;
		width: 4em;
		border-top: thin solid;
	}

/** Data tables (and properly marked-up index tables) *************************/
	/*
		 <table class="data"> highlights structural relationships in a table
		 when correct markup is used (e.g. thead/tbody, th vs. td, scope attribute)

		 Use class="complex data" for particularly complicated tables --
		 (This will draw more lines: busier, but clearer.)

		 Use class="long" on table cells with paragraph-like contents
		 (This will adjust text alignment accordingly.)
		 Alternately use class="longlastcol" on tables, to have the last column assume "long".
	*/

	table {
		word-wrap: normal;
		overflow-wrap: normal;
		hyphens: manual;
	}

	table.data,
	table.index {
		margin: 1em auto;
		border-collapse: collapse;
		border: hidden;
		width: 100%;
	}
	table.data caption,
	table.index caption {
		max-width: 50em;
		margin: 0 auto 1em;
	}

	table.data td,  table.data th,
	table.index td, table.index th {
		padding: 0.5em 1em;
		border-width: 1px;
		border-color: silver;
		border-top-style: solid;
	}

	table.data thead td:empty {
		padding: 0;
		border: 0;
	}

	table.data  thead,
	table.index thead,
	table.data  tbody,
	table.index tbody {
		border-bottom: 2px solid;
	}

	table.data colgroup,
	table.index colgroup {
		border-left: 2px solid;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		border-right: 2px solid;
		border-top: 1px solid silver;
		padding-right: 1em;
	}

	table.data th[colspan],
	table.data td[colspan] {
		text-align: center;
	}

	table.complex.data th,
	table.complex.data td {
		border: 1px solid silver;
		text-align: center;
	}

	table.data.longlastcol td:last-child,
	table.data td.long {
	 vertical-align: baseline;
	 text-align: left;
	}

	table.data img {
		vertical-align: middle;
	}


/*
Alternate table alignment rules

	table.data,
	table.index {
		text-align: center;
	}

	table.data  thead th[scope="row"],
	table.index thead th[scope="row"] {
		text-align: right;
	}

	table.data  tbody th:first-child,
	table.index tbody th:first-child  {
		text-align: right;
	}

Possible extra rowspan handling

	table.data  tbody th[rowspan]:not([rowspan='1']),
	table.index tbody th[rowspan]:not([rowspan='1']),
	table.data  tbody td[rowspan]:not([rowspan='1']),
	table.index tbody td[rowspan]:not([rowspan='1']) {
		border-left: 1px solid silver;
	}

	table.data  tbody th[rowspan]:first-child,
	table.index tbody th[rowspan]:first-child,
	table.data  tbody td[rowspan]:first-child,
	table.index tbody td[rowspan]:first-child{
		border-left: 0;
		border-right: 1px solid silver;
	}
*/

/******************************************************************************/
/*                                  Indices                                   */
/******************************************************************************/


/** Table of Contents *********************************************************/

	.toc a {
		/* More spacing; use padding to make it part of the click target. */
		padding-top: 0.1rem;
		/* Larger, more consistently-sized click target */
		display: block;
		/* Reverse color scheme */
		color: black;
		border-color: #3980B5;
		border-bottom-width: 3px !important;
		margin-bottom: 0px !important;
	}
	.toc a:visited {
		border-color: #054572;
	}
	.toc a:not(:focus):not(:hover) {
		/* Allow colors to cascade through from link styling */
		border-bottom-color: transparent;
	}

	.toc, .toc ol, .toc ul, .toc li {
		list-style: none; /* Numbers must be inlined into source */
		/* because generated content isn't search/selectable and markers can't do multilevel yet */
		margin:  0;
		padding: 0;
		line-height: 1.1rem; /* consistent spacing */
	}

	/* ToC not indented until third level, but font style & margins show hierarchy */
	.toc > li             { font-weight: bold;   }
	.toc > li li          { font-weight: normal; }
	.toc > li li li       { font-size:   95%;    }
	.toc > li li li li    { font-size:   90%;    }
	.toc > li li li li .secno { font-size: 85%; }
	.toc > li li li li li { font-size:   85%;    }
	.toc > li li li li li .secno { font-size: 100%; }

	/* @supports not (display:grid) { */
		.toc > li             { margin: 1.5rem 0;    }
		.toc > li li          { margin: 0.3rem 0;    }
		.toc > li li li       { margin-left: 2rem;   }

		/* Section numbers in a column of their own */
		.toc .secno {
			float: left;
			width: 4rem;
			white-space: nowrap;
		}

		.toc li {
			clear: both;
		}

		:not(li) > .toc              { margin-left:  5rem; }
		.toc .secno                  { margin-left: -5rem; }
		.toc > li li li .secno       { margin-left: -7rem; }
		.toc > li li li li .secno    { margin-left: -9rem; }
		.toc > li li li li li .secno { margin-left: -11rem; }

		/* Tighten up indentation in narrow ToCs */
		@media (max-width: 30em) {
			:not(li) > .toc              { margin-left:  4rem; }
			.toc .secno                  { margin-left: -4rem; }
			.toc > li li li              { margin-left:  1rem; }
			.toc > li li li .secno       { margin-left: -5rem; }
			.toc > li li li li .secno    { margin-left: -6rem; }
			.toc > li li li li li .secno { margin-left: -7rem; }
		}
	/* } */

	@supports (display:grid) and (display:contents) {
		/* Use #toc over .toc to override non-@supports rules. */
		#toc {
			display: grid;
			align-content: start;
			grid-template-columns: auto 1fr;
			grid-column-gap: 1rem;
			column-gap: 1rem;
			grid-row-gap: .6rem;
			row-gap: .6rem;
		}
		#toc h2 {
			grid-column: 1 / -1;
			margin-bottom: 0;
		}
		#toc ol,
		#toc li,
		#toc a {
			display: contents;
			/* Switch <a> to subgrid when supported */
		}
		#toc span {
			margin: 0;
		}
		#toc > .toc > li > a > span {
			/* The spans of the top-level list,
			   comprising the first items of each top-level section. */
			margin-top: 1.1rem;
		}
		#toc#toc .secno { /* Ugh, need more specificity to override base.css */
			grid-column: 1;
			width: auto;
			margin-left: 0;
		}
		#toc .content {
			grid-column: 2;
			width: auto;
			margin-right: 1rem;
		}
		#toc .content:hover {
			background: rgba(75%, 75%, 75%, .25);
			border-bottom: 3px solid #054572;
			margin-bottom: -3px;
		}
		#toc li li li .content {
			margin-left: 1rem;
		}
		#toc li li li li .content {
			margin-left: 2rem;
		}
	}


/** Index *********************************************************************/

	/* Index Lists: Layout */
	ul.index       { margin-left: 0; columns: 15em; text-indent: 1em hanging; }
	ul.index li    { margin-left: 0; list-style: none; break-inside: avoid; }
	ul.index li li { margin-left: 1em }
	ul.index dl    { margin-top: 0; }
	ul.index dt    { margin: .2em 0 .2em 20px;}
	ul.index dd    { margin: .2em 0 .2em 40px;}
	/* Index Lists: Typography */
	ul.index ul,
	ul.index dl { font-size: smaller; }
	@media not print {
		ul.index li span {
			white-space: nowrap;
			color: transparent; }
		ul.index li a:hover + span,
		ul.index li a:focus + span {
			color: #707070;
		}
	}

/** Index Tables *****************************************************/
	/* See also the data table styling section, which this effectively subclasses */

	table.index {
		font-size: small;
		border-collapse: collapse;
		border-spacing: 0;
		text-align: left;
		margin: 1em 0;
	}

	table.index td,
	table.index th {
		padding: 0.4em;
	}

	table.index tr:hover td:not([rowspan]),
	table.index tr:hover th:not([rowspan]) {
		background: #f7f8f9;
	}

	/* The link in the first column in the property table (formerly a TD) */
	table.index th:first-child a {
		font-weight: bold;
	}

/******************************************************************************/
/*                                    Print                                   */
/******************************************************************************/

	@media print {
		/* Pages have their own margins. */
		html {
			margin: 0;
		}
		/* Serif for print. */
		body {
			font-family: serif;
		}
	}
	@page {
		margin: 1.5cm 1.1cm;
	}

/******************************************************************************/
/*                                    Legacy                                  */
/******************************************************************************/

	/* This rule is inherited from past style sheets. No idea what it's for. */
	.hide { display: none }



/******************************************************************************/
/*                             Overflow Control                               */
/******************************************************************************/

	.figure .caption, .sidefigure .caption, figcaption {
		/* in case figure is overlarge, limit caption to 50em */
		max-width: 50rem;
		margin-left: auto;
		margin-right: auto;
	}
	.overlarge {
		/* Magic to create good table positioning:
		   "content column" is 50ems wide at max; less on smaller screens.
		   Extra space (after ToC + content) is empty on the right.

		   1. When table < content column, centers table in column.
		   2. When content < table < available, left-aligns.
		   3. When table > available, fills available + scroll bar.
		*/ 
		display: grid;
		grid-template-columns: minmax(0, 50em);
	}
	.overlarge > table {
		/* limit preferred width of table */
		max-width: 50em;
		margin-left: auto;
		margin-right: auto;
	}

	@media (min-width: 55em) {
		.overlarge {
			margin-right: calc(13px + 26.5rem - 50vw);
			max-width: none;
		}
	}
	@media screen and (min-width: 78em) {
		body:not(.toc-inline) .overlarge {
			/* 30.5em body padding 50em content area */
			margin-right: calc(40em - 50vw) !important;
		}
	}
	@media screen and (min-width: 90em) {
		body:not(.toc-inline) .overlarge {
			/* 4em html margin 30.5em body padding 50em content area */
			margin-right: calc(84.5em - 100vw) !important;
		}
	}

	@media not print {
		.overlarge {
			overflow-x: auto;
			/* See Lea Verou's explanation background-attachment:
			 * http://lea.verou.me/2012/04/background-attachment-local/
			 *
			background: top left  / 4em 100% linear-gradient(to right,  #ffffff, rgba(255, 255, 255, 0)) local,
			            top right / 4em 100% linear-gradient(to left, #ffffff, rgba(255, 255, 255, 0)) local,
			            top left  / 1em 100% linear-gradient(to right,  #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            top right / 1em 100% linear-gradient(to left, #c3c3c5, rgba(195, 195, 197, 0)) scroll,
			            white;
			background-repeat: no-repeat;
			*/
		}
	}
</style>
  <link href="https://www.w3.org/StyleSheets/TR/2016/cg-draft" rel="stylesheet">
  <link href="https://wicg.github.io/BackgroundSync/spec/" rel="canonical">
<style>/* style-md-lists */

/* This is a weird hack for me not yet following the commonmark spec
   regarding paragraph and lists. */
[data-md] > :first-child {
    margin-top: 0;
}
[data-md] > :last-child {
    margin-bottom: 0;
}</style>
<style>/* style-selflinks */

.heading, .issue, .note, .example, li, dt {
    position: relative;
}
a.self-link {
    position: absolute;
    top: 0;
    left: calc(-1 * (3.5rem - 26px));
    width: calc(3.5rem - 26px);
    height: 2em;
    text-align: center;
    border: none;
    transition: opacity .2s;
    opacity: .5;
}
a.self-link:hover {
    opacity: 1;
}
.heading > a.self-link {
    font-size: 83%;
}
li > a.self-link {
    left: calc(-1 * (3.5rem - 26px) - 2em);
}
dfn > a.self-link {
    top: auto;
    left: auto;
    opacity: 0;
    width: 1.5em;
    height: 1.5em;
    background: gray;
    color: white;
    font-style: normal;
    transition: opacity .2s, background-color .2s, color .2s;
}
dfn:hover > a.self-link {
    opacity: 1;
}
dfn > a.self-link:hover {
    color: black;
}

a.self-link::before            { content: "¶"; }
.heading > a.self-link::before { content: "§"; }
dfn > a.self-link::before      { content: "#"; }</style>
<style>/* style-counters */

body {
    counter-reset: example figure issue;
}
.issue {
    counter-increment: issue;
}
.issue:not(.no-marker)::before {
    content: "Issue " counter(issue);
}

.example {
    counter-increment: example;
}
.example:not(.no-marker)::before {
    content: "Example " counter(example);
}
.invalid.example:not(.no-marker)::before,
.illegal.example:not(.no-marker)::before {
    content: "Invalid Example" counter(example);
}

figcaption {
    counter-increment: figure;
}
figcaption:not(.no-marker)::before {
    content: "Figure " counter(figure) " ";
}</style>
<style>/* style-autolinks */

.css.css, .property.property, .descriptor.descriptor {
    color: #005a9c;
    font-size: inherit;
    font-family: inherit;
}
.css::before, .property::before, .descriptor::before {
    content: "‘";
}
.css::after, .property::after, .descriptor::after {
    content: "’";
}
.property, .descriptor {
    /* Don't wrap property and descriptor names */
    white-space: nowrap;
}
.type { /* CSS value <type> */
    font-style: italic;
}
pre .property::before, pre .property::after {
    content: "";
}
[data-link-type="property"]::before,
[data-link-type="propdesc"]::before,
[data-link-type="descriptor"]::before,
[data-link-type="value"]::before,
[data-link-type="function"]::before,
[data-link-type="at-rule"]::before,
[data-link-type="selector"]::before,
[data-link-type="maybe"]::before {
    content: "‘";
}
[data-link-type="property"]::after,
[data-link-type="propdesc"]::after,
[data-link-type="descriptor"]::after,
[data-link-type="value"]::after,
[data-link-type="function"]::after,
[data-link-type="at-rule"]::after,
[data-link-type="selector"]::after,
[data-link-type="maybe"]::after {
    content: "’";
}

[data-link-type].production::before,
[data-link-type].production::after,
.prod [data-link-type]::before,
.prod [data-link-type]::after {
    content: "";
}

[data-link-type=element],
[data-link-type=element-attr] {
    font-family: Menlo, Consolas, "DejaVu Sans Mono", monospace;
    font-size: .9em;
}
[data-link-type=element]::before { content: "<" }
[data-link-type=element]::after  { content: ">" }

[data-link-type=biblio] {
    white-space: pre;
}</style>
<style>/* style-dfn-panel */

.dfn-panel {
    position: absolute;
    z-index: 35;
    height: auto;
    width: -webkit-fit-content;
    width: fit-content;
    max-width: 300px;
    max-height: 500px;
    overflow: auto;
    padding: 0.5em 0.75em;
    font: small Helvetica Neue, sans-serif, Droid Sans Fallback;
    background: #DDDDDD;
    color: black;
    border: outset 0.2em;
}
.dfn-panel:not(.on) { display: none; }
.dfn-panel * { margin: 0; padding: 0; text-indent: 0; }
.dfn-panel > b { display: block; }
.dfn-panel a { color: black; }
.dfn-panel a:not(:hover) { text-decoration: none !important; border-bottom: none !important; }
.dfn-panel > b + b { margin-top: 0.25em; }
.dfn-panel ul { padding: 0; }
.dfn-panel li { list-style: inside; }
.dfn-panel.activated {
    display: inline-block;
    position: fixed;
    left: .5em;
    bottom: 2em;
    margin: 0 auto;
    max-width: calc(100vw - 1.5em - .4em - .5em);
    max-height: 30vh;
}

.dfn-paneled { cursor: pointer; }
</style>
<style>/* style-syntax-highlighting */
pre.idl.highlight { color: #708090; }
.highlight:not(.idl) { background: hsl(24, 20%, 95%); }
code.highlight { padding: .1em; border-radius: .3em; }
pre.highlight, pre > code.highlight { display: block; padding: 1em; margin: .5em 0; overflow: auto; border-radius: 0; }
c-[a] { color: #990055 } /* Keyword.Declaration */
c-[b] { color: #990055 } /* Keyword.Type */
c-[c] { color: #708090 } /* Comment */
c-[d] { color: #708090 } /* Comment.Multiline */
c-[e] { color: #0077aa } /* Name.Attribute */
c-[f] { color: #669900 } /* Name.Tag */
c-[g] { color: #222222 } /* Name.Variable */
c-[k] { color: #990055 } /* Keyword */
c-[l] { color: #000000 } /* Literal */
c-[m] { color: #000000 } /* Literal.Number */
c-[n] { color: #0077aa } /* Name */
c-[o] { color: #999999 } /* Operator */
c-[p] { color: #999999 } /* Punctuation */
c-[s] { color: #a67f59 } /* Literal.String */
c-[t] { color: #a67f59 } /* Literal.String.Single */
c-[u] { color: #a67f59 } /* Literal.String.Double */
c-[cp] { color: #708090 } /* Comment.Preproc */
c-[c1] { color: #708090 } /* Comment.Single */
c-[cs] { color: #708090 } /* Comment.Special */
c-[kc] { color: #990055 } /* Keyword.Constant */
c-[kn] { color: #990055 } /* Keyword.Namespace */
c-[kp] { color: #990055 } /* Keyword.Pseudo */
c-[kr] { color: #990055 } /* Keyword.Reserved */
c-[ld] { color: #000000 } /* Literal.Date */
c-[nc] { color: #0077aa } /* Name.Class */
c-[no] { color: #0077aa } /* Name.Constant */
c-[nd] { color: #0077aa } /* Name.Decorator */
c-[ni] { color: #0077aa } /* Name.Entity */
c-[ne] { color: #0077aa } /* Name.Exception */
c-[nf] { color: #0077aa } /* Name.Function */
c-[nl] { color: #0077aa } /* Name.Label */
c-[nn] { color: #0077aa } /* Name.Namespace */
c-[py] { color: #0077aa } /* Name.Property */
c-[ow] { color: #999999 } /* Operator.Word */
c-[mb] { color: #000000 } /* Literal.Number.Bin */
c-[mf] { color: #000000 } /* Literal.Number.Float */
c-[mh] { color: #000000 } /* Literal.Number.Hex */
c-[mi] { color: #000000 } /* Literal.Number.Integer */
c-[mo] { color: #000000 } /* Literal.Number.Oct */
c-[sb] { color: #a67f59 } /* Literal.String.Backtick */
c-[sc] { color: #a67f59 } /* Literal.String.Char */
c-[sd] { color: #a67f59 } /* Literal.String.Doc */
c-[se] { color: #a67f59 } /* Literal.String.Escape */
c-[sh] { color: #a67f59 } /* Literal.String.Heredoc */
c-[si] { color: #a67f59 } /* Literal.String.Interpol */
c-[sx] { color: #a67f59 } /* Literal.String.Other */
c-[sr] { color: #a67f59 } /* Literal.String.Regex */
c-[ss] { color: #a67f59 } /* Literal.String.Symbol */
c-[vc] { color: #0077aa } /* Name.Variable.Class */
c-[vg] { color: #0077aa } /* Name.Variable.Global */
c-[vi] { color: #0077aa } /* Name.Variable.Instance */
c-[il] { color: #000000 } /* Literal.Number.Integer.Long */
</style>
 <body class="h-entry">
  <div class="head">
   <p data-fill-with="logo"></p>
   <h1 class="p-name no-ref" id="title">Web Background Synchronization</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Draft Community Group Report, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="https://wicg.github.io/BackgroundSync/spec/">https://wicg.github.io/BackgroundSync/spec/</a>
     <dt class="editor">Editors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:jkarlin@chromium.org">Josh Karlin</a> (<span class="p-org org">Google</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:mek@chromium.org">Marijn Kruisselbrink</a> (<span class="p-org org">Google</span>)
    </dl>
   </div>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 the Contributors to the Web Background Synchronization Specification, published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a> under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a>.
A human-readable <a href="http://www.w3.org/community/about/agreements/cla-deed/">summary</a> is available. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This specification describes a method that enables web applications to synchronize data in the background.</p>
  </div>
  <div data-fill-with="at-risk"></div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This specification was published by the <a href="https://www.w3.org/community/wicg/">Web Platform Incubator Community Group</a>.
  It is not a W3C Standard nor is it on the W3C Standards Track.

  Please note that under the <a href="https://www.w3.org/community/about/agreements/cla/">W3C Community Contributor License Agreement (CLA)</a> there is a limited opt-out and other conditions apply.

  Learn more about <a href="http://www.w3.org/community/">W3C Community and Business Groups</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li><a href="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#concepts"><span class="secno">2</span> <span class="content">Concepts</span></a>
    <li><a href="#constructs"><span class="secno">3</span> <span class="content">Constructs</span></a>
    <li>
     <a href="#privacy-considerations"><span class="secno">4</span> <span class="content">Privacy Considerations</span></a>
     <ol class="toc">
      <li><a href="#permission"><span class="secno">4.1</span> <span class="content">Permission</span></a>
      <li><a href="#location-tracking"><span class="secno">4.2</span> <span class="content">Location Tracking</span></a>
      <li><a href="#history-leaking"><span class="secno">4.3</span> <span class="content">History Leaking</span></a>
     </ol>
    <li>
     <a href="#api-description"><span class="secno">5</span> <span class="content">API Description</span></a>
     <ol class="toc">
      <li><a href="#service-worker-registration-extensions"><span class="secno">5.1</span> <span class="content">Extensions to the <code class="idl"><span>ServiceWorkerRegistration</span></code> interface</span></a>
      <li><a href="#sync-manager-interface"><span class="secno">5.2</span> <span class="content"><code class="idl"><span>SyncManager</span></code> interface</span></a>
      <li><a href="#sync-event"><span class="secno">5.3</span> <span class="content">The sync event</span></a>
     </ol>
    <li><a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
     </ol>
    <li><a href="#idl-index"><span class="secno"></span> <span class="content">IDL Index</span></a>
   </ol>
  </nav>
  <main>
   <section>
    <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>
    <p><em>This section is non-normative.</em></p>
    <p>Web Applications often run in environments with unreliable networks (e.g., mobile phones) and unknown lifetimes (the browser might be killed or the user might navigate away). This makes it difficult to synchronize client data from web apps (such as photo uploads, document changes, or composed emails) with servers. If the browser closes or the user navigates away before synchronization can complete, the app must wait until the user revisits the page to try again. This specification provides a new onsync <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept" id="ref-for-service-worker-concept">service worker</a> event which can fire <a data-link-type="dfn" href="#in-the-background" id="ref-for-in-the-background">in the background</a> so that synchronization attempts can continue despite adverse conditions when initially requested. This API is intended to reduce the time between content creation and content synchronization with the server.</p>
    <p>As this API relies on service workers, functionality provided by this API is only available in a <a data-link-type="dfn" href="https://w3c.github.io/webappsec/specs/powerfulfeatures/#secure-context" id="ref-for-secure-context">secure context</a>.</p>
    <div class="example" id="example-873b4e55">
     <a class="self-link" href="#example-873b4e55"></a> Requesting a background sync opportunity from a <a data-link-type="dfn" href="https://html.spec.whatwg.org/#browsing-context" id="ref-for-browsing-context">browsing context</a>: 
<pre class="lang-js highlight"><c- a>function</c-> sendChatMessage<c- p>(</c->message<c- p>)</c-> <c- p>{</c->
  <c- k>return</c-> addChatMessageToOutbox<c- p>(</c->message<c- p>).</c->then<c- p>(()</c-> <c- p>=></c-> <c- p>{</c->
    <c- c1>// Wait for the scoped service worker registration to get a</c->
    <c- c1>// service worker with an active state</c->
    <c- k>return</c-> navigator<c- p>.</c->serviceWorker<c- p>.</c->ready<c- p>;</c->
  <c- p>}).</c->then<c- p>(</c->reg <c- p>=></c-> <c- p>{</c->
    <c- k>return</c-> reg<c- p>.</c->sync<c- p>.</c->register<c- p>(</c-><c- t>'send-chats'</c-><c- p>);</c->
  <c- p>}).</c->then<c- p>(()</c-> <c- p>=></c-> <c- p>{</c->
    console<c- p>.</c->log<c- p>(</c-><c- t>'Sync registered!'</c-><c- p>);</c->
  <c- p>}).</c-><c- k>catch</c-><c- p>(()</c-> <c- p>=></c-> <c- p>{</c->
    console<c- p>.</c->log<c- p>(</c-><c- t>'Sync registration failed :('</c-><c- p>);</c->
  <c- p>});</c->
<c- p>}</c->
</pre>
     <p>In the above example <code>addChatMessageToOutbox</code> is a developer-defined function.</p>
     <p>Reacting to a sync event within a <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept" id="ref-for-service-worker-concept①">service worker</a>:</p>
<pre class="lang-js highlight">self<c- p>.</c->addEventListener<c- p>(</c-><c- t>'sync'</c-><c- p>,</c-> event <c- p>=></c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c->event<c- p>.</c->tag <c- o>==</c-> <c- t>'send-chats'</c-><c- p>)</c-> <c- p>{</c->
    event<c- p>.</c->waitUntil<c- p>(</c->
      getMessagesFromOutbox<c- p>().</c->then<c- p>(</c->messages <c- p>=></c-> <c- p>{</c->
        <c- c1>// Post the messages to the server</c->
        <c- k>return</c-> fetch<c- p>(</c-><c- t>'/send'</c-><c- p>,</c-> <c- p>{</c->
          method<c- o>:</c-> <c- t>'POST'</c-><c- p>,</c->
          body<c- o>:</c-> JSON<c- p>.</c->stringify<c- p>(</c->messages<c- p>),</c->
          headers<c- o>:</c-> <c- p>{</c-> <c- t>'Content-Type'</c-><c- o>:</c-> <c- t>'application/json'</c-> <c- p>}</c->
        <c- p>}).</c->then<c- p>(()</c-> <c- p>=></c-> <c- p>{</c->
          <c- c1>// Success! Remove them from the outbox</c->
          <c- k>return</c-> removeMessagesFromOutbox<c- p>(</c->messages<c- p>);</c->
        <c- p>});</c->
      <c- p>}).</c->then<c- p>(()</c-> <c- p>=></c-> <c- p>{</c->
        <c- c1>// Tell pages of your success so they can update UI</c->
        <c- k>return</c-> clients<c- p>.</c->matchAll<c- p>({</c-> includeUncontrolled<c- o>:</c-> <c- kc>true</c-> <c- p>});</c->
      <c- p>}).</c->then<c- p>(</c->clients <c- p>=></c-> <c- p>{</c->
        clients<c- p>.</c->forEach<c- p>(</c->client <c- p>=></c-> client<c- p>.</c->postMessage<c- p>(</c-><c- t>'outbox-processed'</c-><c- p>))</c->
      <c- p>})</c->
    <c- p>);</c->
  <c- p>}</c->
<c- p>});</c->
</pre>
     <p>In the above example <code>getMessagesFromOutbox</code> and <code>removeMessagesFromOutbox</code> are developer-defined functions.</p>
    </div>
   </section>
   <section>
    <h2 class="heading settled" data-level="2" id="concepts"><span class="secno">2. </span><span class="content">Concepts</span><a class="self-link" href="#concepts"></a></h2>
    <p>The sync event is considered to run <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="in-the-background">in the background</dfn> if no <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client" id="ref-for-dfn-service-worker-client">service worker clients</a> whose <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client-frame-type" id="ref-for-dfn-service-worker-client-frame-type">frame type</a> is top-level or auxiliary exist for the origin of the corresponding service worker registration.</p>
    <p>The user agent is considered to be <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="online">online</dfn> if the user agent has established a network connection. A user agent MAY use a stricter definition of being <a data-link-type="dfn" href="#online" id="ref-for-online">online</a>. Such a stricter definition MAY take into account the particular <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept" id="ref-for-service-worker-concept②">service worker</a> or origin a <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration">sync registration</a> is associated with.</p>
   </section>
   <section>
    <h2 class="heading settled" data-level="3" id="constructs"><span class="secno">3. </span><span class="content">Constructs</span><a class="self-link" href="#constructs"></a></h2>
     A <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept">service worker registration</a> has an associated <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="list-of-sync-registrations">list of sync registrations</dfn> whose element type is a <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration①">sync registration</a>. 
    <p>A <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="sync-registration">sync registration</dfn> is a tuple consisting of a <a data-link-type="dfn" href="#tag" id="ref-for-tag">tag</a> and a <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state">state</a>.</p>
    <p>A <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration②">sync registration</a> has an associated <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="tag">tag</dfn>, a DOMString.</p>
    <p>A <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration③">sync registration</a> has an associated <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="registration-state">registration state</dfn>, which is one of <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="pending">pending</dfn>, <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="waiting">waiting</dfn>, <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="firing">firing</dfn>, or <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="reregisteredwhilefiring">reregisteredWhileFiring</dfn>. It is initially set to <a data-link-type="dfn" href="#pending" id="ref-for-pending">pending</a>.</p>
    <p>A <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration④">sync registration</a> has an associated <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept①">service worker registration</a>. It is initially set to null.</p>
    <p>Within one <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations">list of sync registrations</a> each <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration⑤">sync registration</a> MUST have a unique <a data-link-type="dfn" href="#tag" id="ref-for-tag①">tag</a>.</p>
   </section>
   <section>
    <h2 class="heading settled" data-level="4" id="privacy-considerations"><span class="secno">4. </span><span class="content">Privacy Considerations</span><a class="self-link" href="#privacy-considerations"></a></h2>
    <section>
     <h3 class="heading settled" data-level="4.1" id="permission"><span class="secno">4.1. </span><span class="content">Permission</span><a class="self-link" href="#permission"></a></h3>
      User agents MAY offer a way for the user to disable background sync. 
     <p class="note" role="note"><span>Note:</span> Background sync SHOULD be enabled by default. Having the permission denied is considered an exceptional case.</p>
    </section>
    <section>
     <h3 class="heading settled" data-level="4.2" id="location-tracking"><span class="secno">4.2. </span><span class="content">Location Tracking</span><a class="self-link" href="#location-tracking"></a></h3>
      Fetch requests within the onsync event while <a data-link-type="dfn" href="#in-the-background" id="ref-for-in-the-background①">in the background</a> may reveal the client’s IP address to the server after the user left the page. The user agent SHOULD limit tracking by capping the number of retries and duration of sync events. 
    </section>
    <section>
     <h3 class="heading settled" data-level="4.3" id="history-leaking"><span class="secno">4.3. </span><span class="content">History Leaking</span><a class="self-link" href="#history-leaking"></a></h3>
      Fetch requests within the onsync event while <a data-link-type="dfn" href="#in-the-background" id="ref-for-in-the-background②">in the background</a> may reveal something about the client’s navigation history to passive eavesdroppers. For instance, the client might visit site https://example.com, which registers a sync event, but doesn’t fire until after the user has navigated away from the page and changed networks. Passive eavesdroppers on the new network may see the fetch requests that the onsync event makes. The fetch requests are HTTPS so the request contents will not be leaked but the domain may be (via DNS lookups and IP address of the request). 
    </section>
   </section>
   <section>
    <h2 class="heading settled" data-level="5" id="api-description"><span class="secno">5. </span><span class="content">API Description</span><a class="self-link" href="#api-description"></a></h2>
    <section>
     <h3 class="heading settled" data-level="5.1" id="service-worker-registration-extensions"><span class="secno">5.1. </span><span class="content">Extensions to the <code class="idl"><a data-link-type="idl" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface" id="ref-for-service-worker-registration-interface">ServiceWorkerRegistration</a></code> interface</span><a class="self-link" href="#service-worker-registration-extensions"></a></h3>
<pre class="idl highlight def"><c- b>partial</c-> <c- b>interface</c-> <a class="idl-code" data-link-type="interface" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface" id="ref-for-service-worker-registration-interface①"><c- g>ServiceWorkerRegistration</c-></a> {
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="n" data-link-type="idl-name" href="#syncmanager" id="ref-for-syncmanager"><c- n>SyncManager</c-></a> <dfn class="idl-code" data-dfn-for="ServiceWorkerRegistration" data-dfn-type="attribute" data-export data-readonly data-type="SyncManager" id="dom-serviceworkerregistration-sync"><code><c- g>sync</c-></code><a class="self-link" href="#dom-serviceworkerregistration-sync"></a></dfn>;
};
</pre>
     <p>The <code><dfn class="idl-code" data-dfn-for="SyncManager" data-dfn-type="attribute" data-export id="dom-syncmanager-sync" title="sync"><code>sync</code><a class="self-link" href="#dom-syncmanager-sync"></a></dfn></code> attribute exposes a <code class="idl"><a data-link-type="idl" href="#syncmanager" id="ref-for-syncmanager①">SyncManager</a></code>, which has an associated <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept②">service worker registration</a> represented by the <code class="idl"><a data-link-type="idl" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface" id="ref-for-service-worker-registration-interface②">ServiceWorkerRegistration</a></code> on which the attribute is exposed.</p>
    </section>
    <section>
     <h3 class="heading settled" data-level="5.2" id="sync-manager-interface"><span class="secno">5.2. </span><span class="content"><code class="idl"><a data-link-type="idl" href="#syncmanager" id="ref-for-syncmanager②">SyncManager</a></code> interface</span><a class="self-link" href="#sync-manager-interface"></a></h3>
<pre class="idl highlight def">[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed"><c- g>Exposed</c-></a>=(<c- n>Window</c->,<c- n>Worker</c->)]
<c- b>interface</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export id="syncmanager"><code><c- g>SyncManager</c-></code></dfn> {
  <c- b>Promise</c->&lt;<c- b>void</c->> <a class="idl-code" data-link-type="method" href="#dom-syncmanager-register" id="ref-for-dom-syncmanager-register"><c- g>register</c-></a>(<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString"><c- b>DOMString</c-></a> <dfn class="idl-code" data-dfn-for="SyncManager/register(tag)" data-dfn-type="argument" data-export id="dom-syncmanager-register-tag-tag"><code><c- g>tag</c-></code><a class="self-link" href="#dom-syncmanager-register-tag-tag"></a></dfn>);
  <c- b>Promise</c->&lt;<c- b>sequence</c->&lt;<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString①"><c- b>DOMString</c-></a>>> <a class="idl-code" data-link-type="method" href="#dom-syncmanager-gettags" id="ref-for-dom-syncmanager-gettags"><c- g>getTags</c-></a>();
};
</pre>
     <p>The <code><dfn class="dfn-paneled idl-code" data-dfn-for="SyncManager" data-dfn-type="method" data-export id="dom-syncmanager-register" title="register(tag)"><code>register(<var>tag</var>)</code></dfn></code> method, when invoked, MUST return <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#a-new-promise" id="ref-for-a-new-promise">a new promise</a> <var>promise</var> and run the following steps <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel" id="ref-for-in-parallel">in parallel</a>:</p>
     <ol>
      <li> Let <var>serviceWorkerRegistration</var> be the <code class="idl"><a data-link-type="idl" href="#syncmanager" id="ref-for-syncmanager③">SyncManager</a></code>'s associated <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept③">service worker registration</a>. 
      <li> If <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-active-worker" id="ref-for-dfn-active-worker">active worker</a> is null, <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#reject-promise" id="ref-for-reject-promise">reject</a> <var>promise</var> with an <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#invalidstateerror" id="ref-for-invalidstateerror">InvalidStateError</a></code> and abort these steps. 
      <li> If the user has disabled background sync, <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#reject-promise" id="ref-for-reject-promise①">reject</a> <var>promise</var> with an <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#notallowederror" id="ref-for-notallowederror">NotAllowedError</a></code> and abort these steps. 
      <li> Let <var>isBackground</var> be true. 
      <li>
        For each <var>client</var> in the <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client" id="ref-for-dfn-service-worker-client①">service worker clients</a> for the <var>serviceWorkerRegistration</var>’s origin: 
       <ol>
        <li> If <var>client</var>’s <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client-frame-type" id="ref-for-dfn-service-worker-client-frame-type①">frame type</a> is top-level or auxiliary, set <var>isBackground</var> to false. 
       </ol>
      <li> If <var>isBackground</var> is true, <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#reject-promise" id="ref-for-reject-promise②">reject</a> <var>promise</var> with an <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#invalidaccesserror" id="ref-for-invalidaccesserror">InvalidAccessError</a></code> and abort these steps. 
      <li> Let <var>currentRegistration</var> be the <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration⑥">registration</a> in <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations①">list of sync registrations</a> whose <a data-link-type="dfn" href="#tag" id="ref-for-tag②">tag</a> equals <var>tag</var> if it exists, else null. 
      <li>
        If <var>currentRegistration</var> is not null: 
       <ol>
        <li> If <var>currentRegistration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state①">registration state</a> is <a data-link-type="dfn" href="#waiting" id="ref-for-waiting">waiting</a>, set <var>currentRegistration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state②">registration state</a> to <a data-link-type="dfn" href="#pending" id="ref-for-pending①">pending</a>. 
        <li> If <var>currentRegistration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state③">registration state</a> is <a data-link-type="dfn" href="#firing" id="ref-for-firing">firing</a>, set <var>currentRegistration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state④">registration state</a> to <a data-link-type="dfn" href="#reregisteredwhilefiring" id="ref-for-reregisteredwhilefiring">reregisteredWhileFiring</a>. 
        <li> <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#resolve-promise" id="ref-for-resolve-promise">Resolve</a> <var>promise</var>. 
        <li> If the user agent is currently <a data-link-type="dfn" href="#online" id="ref-for-online①">online</a> and <var>currentRegistration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state⑤">registration state</a> is <a data-link-type="dfn" href="#pending" id="ref-for-pending②">pending</a>, <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event">fire a sync event</a> for <var>currentRegistration</var>. 
       </ol>
      <li>
        Else: 
       <ol>
        <li> Let <var>newRegistration</var> be a new <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration⑦">sync registration</a>. 
        <li> Set <var>newRegistration</var>’s associated <a data-link-type="dfn" href="#tag" id="ref-for-tag③">tag</a> to <var>tag</var>. 
        <li> Set <var>newRegistration</var>’s associated <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept④">service worker registration</a> to <var>serviceWorkerRegistration</var>. 
        <li> Add <var>newRegistration</var> to <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations②">list of sync registrations</a>. 
        <li> <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#resolve-promise" id="ref-for-resolve-promise①">Resolve</a> <var>promise</var>. 
        <li> If the user agent is currently <a data-link-type="dfn" href="#online" id="ref-for-online②">online</a>, <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event①">fire a sync event</a> for <var>newRegistration</var>. 
       </ol>
     </ol>
     <p>The <code><dfn class="dfn-paneled idl-code" data-dfn-for="SyncManager" data-dfn-type="method" data-export id="dom-syncmanager-gettags" title="getTags()"><code>getTags()</code></dfn></code> method when invoked, MUST return <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#a-new-promise" id="ref-for-a-new-promise①">a new promise</a> <var>promise</var> and run the following steps <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel" id="ref-for-in-parallel①">in parallel</a>:</p>
     <ol>
      <li> Let <var>serviceWorkerRegistration</var> be the <code class="idl"><a data-link-type="idl" href="#syncmanager" id="ref-for-syncmanager④">SyncManager</a></code>'s associated <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept⑤">service worker registration</a>. 
      <li>Let <var>currentTags</var> be a new <code class="idl"><a data-link-type="idl" href="https://heycam.github.io/webidl/#idl-sequence" id="ref-for-idl-sequence">sequence</a></code>.
      <li> For each <var>registration</var> in <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations③">list of sync registrations</a>, add <var>registration</var>’s associated <a data-link-type="dfn" href="#tag" id="ref-for-tag④">tag</a> to <var>currentTags</var>. 
      <li> <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#resolve-promise" id="ref-for-resolve-promise②">Resolve</a> <var>promise</var> with <var>currentTags</var>. 
     </ol>
    </section>
    <section>
     <h3 class="heading settled" data-level="5.3" id="sync-event"><span class="secno">5.3. </span><span class="content">The sync event</span><a class="self-link" href="#sync-event"></a></h3>
<pre class="idl highlight def"><c- b>partial</c-> <c- b>interface</c-> <a class="idl-code" data-link-type="interface" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-global-scope-interface" id="ref-for-service-worker-global-scope-interface"><c- g>ServiceWorkerGlobalScope</c-></a> {
  <c- b>attribute</c-> <a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/webappapis.html#eventhandler" id="ref-for-eventhandler"><c- n>EventHandler</c-></a> <dfn class="idl-code" data-dfn-for="ServiceWorkerGlobalScope" data-dfn-type="attribute" data-export data-type="EventHandler" id="dom-serviceworkerglobalscope-onsync"><code><c- g>onsync</c-></code><a class="self-link" href="#dom-serviceworkerglobalscope-onsync"></a></dfn>;
};

[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed①"><c- g>Exposed</c-></a>=<c- n>ServiceWorker</c->]
<c- b>interface</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="interface" data-export id="syncevent"><code><c- g>SyncEvent</c-></code></dfn> : <a class="n" data-link-type="idl-name" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface" id="ref-for-extendable-event-interface"><c- n>ExtendableEvent</c-></a> {
  <dfn class="idl-code" data-dfn-for="SyncEvent" data-dfn-type="constructor" data-export data-lt="SyncEvent(type, init)|constructor(type, init)" id="dom-syncevent-syncevent"><code><c- g>constructor</c-></code><a class="self-link" href="#dom-syncevent-syncevent"></a></dfn>(<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString②"><c- b>DOMString</c-></a> <dfn class="idl-code" data-dfn-for="SyncEvent/SyncEvent(type, init), SyncEvent/constructor(type, init)" data-dfn-type="argument" data-export id="dom-syncevent-syncevent-type-init-type"><code><c- g>type</c-></code><a class="self-link" href="#dom-syncevent-syncevent-type-init-type"></a></dfn>, <a class="n" data-link-type="idl-name" href="#dictdef-synceventinit" id="ref-for-dictdef-synceventinit"><c- n>SyncEventInit</c-></a> <dfn class="idl-code" data-dfn-for="SyncEvent/SyncEvent(type, init), SyncEvent/constructor(type, init)" data-dfn-type="argument" data-export id="dom-syncevent-syncevent-type-init-init"><code><c- g>init</c-></code><a class="self-link" href="#dom-syncevent-syncevent-type-init-init"></a></dfn>);
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString③"><c- b>DOMString</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="SyncEvent" data-dfn-type="attribute" data-export data-readonly data-type="DOMString" id="dom-syncevent-tag"><code><c- g>tag</c-></code></dfn>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean"><c- b>boolean</c-></a> <dfn class="dfn-paneled idl-code" data-dfn-for="SyncEvent" data-dfn-type="attribute" data-export data-readonly data-type="boolean" id="dom-syncevent-lastchance"><code><c- g>lastChance</c-></code></dfn>;
};

<c- b>dictionary</c-> <dfn class="dfn-paneled idl-code" data-dfn-type="dictionary" data-export id="dictdef-synceventinit"><code><c- g>SyncEventInit</c-></code></dfn> : <a class="n" data-link-type="idl-name" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-init-dictionary" id="ref-for-extendable-event-init-dictionary"><c- n>ExtendableEventInit</c-></a> {
  <c- b>required</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString④"><c- b>DOMString</c-></a> <dfn class="idl-code" data-dfn-for="SyncEventInit" data-dfn-type="dict-member" data-export data-type="DOMString " id="dom-synceventinit-tag"><code><c- g>tag</c-></code><a class="self-link" href="#dom-synceventinit-tag"></a></dfn>;
  <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean①"><c- b>boolean</c-></a> <dfn class="idl-code" data-default="false" data-dfn-for="SyncEventInit" data-dfn-type="dict-member" data-export data-type="boolean " id="dom-synceventinit-lastchance"><code><c- g>lastChance</c-></code><a class="self-link" href="#dom-synceventinit-lastchance"></a></dfn> = <c- b>false</c->;
};
</pre>
     <p class="note" role="note"><span>Note:</span> The <code class="idl"><a data-link-type="idl" href="#syncevent" id="ref-for-syncevent">SyncEvent</a></code> interface represents a firing sync registration. If the page (or worker) that registered the event is running, the user agent will fire the sync event as soon as network connectivity is available. Otherwise, the user agent should run the event at the soonest convenience. If a sync event fails, the user agent may decide to retry it at a time of its choosing. The <code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance">lastChance</a></code> attribute is true if the user agent will not make further attempts to try this sync after the current attempt.</p>
     <div class="example" id="example-1ff428a9">
      <a class="self-link" href="#example-1ff428a9"></a> Reacting to <code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance①">lastChance</a></code>: 
<pre class="lang-js highlight">self<c- p>.</c->addEventListener<c- p>(</c-><c- t>'sync'</c-><c- p>,</c-> event <c- p>=></c-> <c- p>{</c->
  <c- k>if</c-> <c- p>(</c->event<c- p>.</c->tag <c- o>==</c-> <c- t>'important-thing'</c-><c- p>)</c-> <c- p>{</c->
    event<c- p>.</c->waitUntil<c- p>(</c->
      doImportantThing<c- p>().</c-><c- k>catch</c-><c- p>(</c->err <c- p>=></c-> <c- p>{</c->
        <c- k>if</c-> <c- p>(</c->event<c- p>.</c->lastChance<c- p>)</c-> <c- p>{</c->
          self<c- p>.</c->registration<c- p>.</c->showNotification<c- p>(</c-><c- u>"Important thing failed"</c-><c- p>);</c->
        <c- p>}</c->
        <c- k>throw</c-> err<c- p>;</c->
      <c- p>})</c->
    <c- p>);</c->
  <c- p>}</c->
<c- p>});</c->
</pre>
      <p>The above example reacts to <code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance②">lastChance</a></code> by showing a <a href="https://notifications.spec.whatwg.org/#concept-notification">notification</a> to the user. This requires the origin to have <a href="https://notifications.spec.whatwg.org/#permission-model">permission to show notifications</a>.</p>
      <p>In the above example <code>doImportantThing</code> is a developer-defined function.</p>
     </div>
     <p>Whenever the user agent changes to <a data-link-type="dfn" href="#online" id="ref-for-online③">online</a>, the user agent SHOULD <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event②">fire a sync event</a> for each <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration⑧">sync registration</a> whose <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state⑥">registration state</a> is <a data-link-type="dfn" href="#pending" id="ref-for-pending③">pending</a>. The events may be fired in any order.</p>
     <p>To <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="fire-a-sync-event">fire a sync event</dfn> for a <a data-link-type="dfn" href="#sync-registration" id="ref-for-sync-registration⑨">sync registration</a> <var>registration</var>, the user agent MUST run the following steps:</p>
     <ol>
      <li> <a data-link-type="dfn" href="http://www.ecma-international.org/ecma-262/6.0/#sec-algorithm-conventions" id="ref-for-sec-algorithm-conventions">Assert</a>: <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state⑦">registration state</a> is <a data-link-type="dfn" href="#pending" id="ref-for-pending④">pending</a>. 
      <li> Let <var>serviceWorkerRegistration</var> be the <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept" id="ref-for-service-worker-registration-concept⑥">service worker registration</a> associated with <var>registration</var>. 
      <li> <a data-link-type="dfn" href="http://www.ecma-international.org/ecma-262/6.0/#sec-algorithm-conventions" id="ref-for-sec-algorithm-conventions①">Assert</a>: <var>registration</var> exists in the <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations④">list of sync registrations</a> associated with <var>serviceWorkerRegistration</var>. 
      <li> Set <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state⑧">registration state</a> to <a data-link-type="dfn" href="#firing" id="ref-for-firing①">firing</a>. 
      <li>
       <p><a data-link-type="dfn" href="https://w3c.github.io/ServiceWorker/#fire-functional-event" id="ref-for-fire-functional-event">Fire Functional Event</a> "<code>sync</code>" using <code class="idl"><a data-link-type="idl" href="#syncevent" id="ref-for-syncevent①">SyncEvent</a></code> on <var>serviceWorkerRegistration</var> with the following properties:</p>
       <dl>
        <dt><code class="idl"><a data-link-type="idl" href="#dom-syncevent-tag" id="ref-for-dom-syncevent-tag">tag</a></code>
        <dd>The <a data-link-type="dfn" href="#tag" id="ref-for-tag⑤">tag</a> associated with <var>registration</var>
        <dt><code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance③">lastChance</a></code>
        <dd>False if the user agent <a data-link-type="dfn" href="#will-retry" id="ref-for-will-retry">will retry</a> this sync event if it fails, or true if no further attempts will be made after the current attempt.
       </dl>
       <p>Then run the following steps with <var>dispatchedEvent</var>:</p>
       <ol>
        <li> Let <var>waitUntilPromise</var> be the result of <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#waiting-for-all" id="ref-for-waiting-for-all">waiting for all</a> of <var>dispatchedEvent</var>’s <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-extend-lifetime-promises" id="ref-for-dfn-extend-lifetime-promises">extended lifetime promises</a>. 
        <li>
          <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#upon-fulfillment" id="ref-for-upon-fulfillment">Upon fulfillment</a> of <var>waitUntilPromise</var>, perform the following steps atomically: 
         <ol>
          <li>
            If <var>registration</var>’s state is <a data-link-type="dfn" href="#reregisteredwhilefiring" id="ref-for-reregisteredwhilefiring①">reregisteredWhileFiring</a>: 
           <ol>
            <li> Set <var>registration</var>’s state to <a data-link-type="dfn" href="#pending" id="ref-for-pending⑤">pending</a>. 
            <li> If the user agent is currently <a data-link-type="dfn" href="#online" id="ref-for-online④">online</a>, <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event③">fire a sync event</a> for <var>registration</var>. 
            <li> Abort the rest of these steps. 
           </ol>
          <li> <a data-link-type="dfn" href="http://www.ecma-international.org/ecma-262/6.0/#sec-algorithm-conventions" id="ref-for-sec-algorithm-conventions②">Assert</a>: <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state⑨">registration state</a> is <a data-link-type="dfn" href="#firing" id="ref-for-firing②">firing</a>. 
          <li> Remove <var>registration</var> from <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations⑤">list of sync registration</a>. 
         </ol>
        <li>
          <a data-link-type="dfn" href="https://www.w3.org/2001/tag/doc/promises-guide#upon-rejection" id="ref-for-upon-rejection">Upon rejection</a> of <var>waitUntilPromise</var>, or if the script has been aborted by the <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#terminate-service-worker-algorithm" id="ref-for-terminate-service-worker-algorithm">termination</a> of the <a data-link-type="dfn" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept" id="ref-for-service-worker-concept③">service worker</a>, perform the following steps atomically: 
         <ol>
          <li>
            If <var>registration</var>’s state is <a data-link-type="dfn" href="#reregisteredwhilefiring" id="ref-for-reregisteredwhilefiring②">reregisteredWhileFiring</a>: 
           <ol>
            <li> Set <var>registration</var>’s state to <a data-link-type="dfn" href="#pending" id="ref-for-pending⑥">pending</a>. 
            <li> If the user agent is currently <a data-link-type="dfn" href="#online" id="ref-for-online⑤">online</a>, <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event④">fire a sync event</a> for <var>registration</var>. 
            <li> Abort the rest of these steps. 
           </ol>
          <li>
            If the <code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance④">lastChance</a></code> attribute of <var>dispatchedEvent</var> is false, set <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state①⓪">registration state</a> to <a data-link-type="dfn" href="#waiting" id="ref-for-waiting①">waiting</a>, and perform the following steps <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel" id="ref-for-in-parallel②">in parallel</a>: 
           <ol>
            <li>Wait a user agent defined length of time.
            <li>If <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state①①">registration state</a> is not <a data-link-type="dfn" href="#waiting" id="ref-for-waiting②">waiting</a>, abort these substeps.
            <li>Set <var>registration</var>’s <a data-link-type="dfn" href="#registration-state" id="ref-for-registration-state①②">registration state</a> to <a data-link-type="dfn" href="#pending" id="ref-for-pending⑦">pending</a>.
            <li>If the user agent is currently <a data-link-type="dfn" href="#online" id="ref-for-online⑥">online</a>, <a data-link-type="dfn" href="#fire-a-sync-event" id="ref-for-fire-a-sync-event⑤">fire a sync event</a> for <var>registration</var>.
           </ol>
          <li> Else remove <var>registration</var> from <var>serviceWorkerRegistration</var>’s <a data-link-type="dfn" href="#list-of-sync-registrations" id="ref-for-list-of-sync-registrations⑥">list of sync registrations</a>. 
         </ol>
       </ol>
     </ol>
     <p>A user agent MAY impose a time limit on the lifetime extension and execution time of a <code class="idl"><a data-link-type="idl" href="#syncevent" id="ref-for-syncevent②">SyncEvent</a></code> which is stricter than the time limit imposed for <code class="idl"><a data-link-type="idl" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface" id="ref-for-extendable-event-interface①">ExtendableEvent</a></code>s in general. In particular an event for which <code class="idl"><a data-link-type="idl" href="#dom-syncevent-lastchance" id="ref-for-dom-syncevent-lastchance⑤">lastChance</a></code> is true MAY have a significantly shortened time limit.</p>
     <p>A user agent <dfn class="dfn-paneled" data-dfn-type="dfn" data-noexport id="will-retry">will retry</dfn> a <a href="#sync-event">sync event</a> based on some user agent defined heuristics.</p>
    </section>
   </section>
  </main>
  <div data-fill-with="conformance">
   <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
   <p> Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology.
            The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL”
            in the normative parts of this document
            are to be interpreted as described in RFC 2119.
            However, for readability,
            these words do not appear in all uppercase letters in this specification. </p>
   <p> All of the text of this specification is normative
            except sections explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
   <p> Examples in this specification are introduced with the words “for example”
            or are set apart from the normative text with <code>class="example"</code>, like this: </p>
   <div class="example" id="example-example"><a class="self-link" href="#example-example"></a> This is an example of an informative example. </div>
   <p> Informative notes begin with the word “Note”
            and are set apart from the normative text with <code>class="note"</code>, like this: </p>
   <p class="note" role="note"> Note, this is an informative note. </p>
  </div>
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  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#dom-syncevent-syncevent">constructor(type, init)</a><span>, in §5.3</span>
   <li><a href="#fire-a-sync-event">fire a sync event</a><span>, in §5.3</span>
   <li><a href="#firing">firing</a><span>, in §3</span>
   <li><a href="#dom-syncmanager-gettags">getTags()</a><span>, in §5.2</span>
   <li><a href="#in-the-background">in the background</a><span>, in §2</span>
   <li>
    lastChance
    <ul>
     <li><a href="#dom-syncevent-lastchance">attribute for SyncEvent</a><span>, in §5.3</span>
     <li><a href="#dom-synceventinit-lastchance">dict-member for SyncEventInit</a><span>, in §5.3</span>
    </ul>
   <li><a href="#list-of-sync-registrations">list of sync registrations</a><span>, in §3</span>
   <li><a href="#online">online</a><span>, in §2</span>
   <li><a href="#dom-serviceworkerglobalscope-onsync">onsync</a><span>, in §5.3</span>
   <li><a href="#pending">pending</a><span>, in §3</span>
   <li><a href="#dom-syncmanager-register">register(tag)</a><span>, in §5.2</span>
   <li><a href="#registration-state">registration state</a><span>, in §3</span>
   <li><a href="#reregisteredwhilefiring">reregisteredWhileFiring</a><span>, in §3</span>
   <li>
    sync
    <ul>
     <li><a href="#dom-serviceworkerregistration-sync">attribute for ServiceWorkerRegistration</a><span>, in §5.1</span>
     <li><a href="#dom-syncmanager-sync">attribute for SyncManager</a><span>, in §5.1</span>
    </ul>
   <li><a href="#syncevent">SyncEvent</a><span>, in §5.3</span>
   <li><a href="#dictdef-synceventinit">SyncEventInit</a><span>, in §5.3</span>
   <li><a href="#dom-syncevent-syncevent">SyncEvent(type, init)</a><span>, in §5.3</span>
   <li><a href="#syncmanager">SyncManager</a><span>, in §5.2</span>
   <li><a href="#sync-registration">sync registration</a><span>, in §3</span>
   <li>
    tag
    <ul>
     <li><a href="#dom-syncevent-tag">attribute for SyncEvent</a><span>, in §5.3</span>
     <li><a href="#tag">definition of</a><span>, in §3</span>
     <li><a href="#dom-synceventinit-tag">dict-member for SyncEventInit</a><span>, in §5.3</span>
    </ul>
   <li><a href="#waiting">waiting</a><span>, in §3</span>
   <li><a href="#will-retry">will retry</a><span>, in §5.3</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-sec-algorithm-conventions">
   <a href="http://www.ecma-international.org/ecma-262/6.0/#sec-algorithm-conventions">http://www.ecma-international.org/ecma-262/6.0/#sec-algorithm-conventions</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-sec-algorithm-conventions">5.3. The sync event</a> <a href="#ref-for-sec-algorithm-conventions①">(2)</a> <a href="#ref-for-sec-algorithm-conventions②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-eventhandler">
   <a href="https://html.spec.whatwg.org/multipage/webappapis.html#eventhandler">https://html.spec.whatwg.org/multipage/webappapis.html#eventhandler</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-eventhandler">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-browsing-context">
   <a href="https://html.spec.whatwg.org/#browsing-context">https://html.spec.whatwg.org/#browsing-context</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-browsing-context">1. Introduction</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-in-parallel">
   <a href="https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel">https://html.spec.whatwg.org/multipage/infrastructure.html#in-parallel</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-in-parallel">5.2. SyncManager interface</a> <a href="#ref-for-in-parallel①">(2)</a>
    <li><a href="#ref-for-in-parallel②">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-a-new-promise">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#a-new-promise">https://www.w3.org/2001/tag/doc/promises-guide#a-new-promise</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-a-new-promise">5.2. SyncManager interface</a> <a href="#ref-for-a-new-promise①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-reject-promise">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#reject-promise">https://www.w3.org/2001/tag/doc/promises-guide#reject-promise</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-reject-promise">5.2. SyncManager interface</a> <a href="#ref-for-reject-promise①">(2)</a> <a href="#ref-for-reject-promise②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-resolve-promise">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#resolve-promise">https://www.w3.org/2001/tag/doc/promises-guide#resolve-promise</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-resolve-promise">5.2. SyncManager interface</a> <a href="#ref-for-resolve-promise①">(2)</a> <a href="#ref-for-resolve-promise②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-upon-fulfillment">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#upon-fulfillment">https://www.w3.org/2001/tag/doc/promises-guide#upon-fulfillment</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-upon-fulfillment">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-upon-rejection">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#upon-rejection">https://www.w3.org/2001/tag/doc/promises-guide#upon-rejection</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-upon-rejection">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-waiting-for-all">
   <a href="https://www.w3.org/2001/tag/doc/promises-guide#waiting-for-all">https://www.w3.org/2001/tag/doc/promises-guide#waiting-for-all</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-waiting-for-all">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-secure-context">
   <a href="https://w3c.github.io/webappsec/specs/powerfulfeatures/#secure-context">https://w3c.github.io/webappsec/specs/powerfulfeatures/#secure-context</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-secure-context">1. Introduction</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-extendable-event-interface">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-extendable-event-interface">5.3. The sync event</a> <a href="#ref-for-extendable-event-interface①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-extendable-event-init-dictionary">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-init-dictionary">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-init-dictionary</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-extendable-event-init-dictionary">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-service-worker-global-scope-interface">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-global-scope-interface">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-global-scope-interface</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-service-worker-global-scope-interface">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-service-worker-registration-interface">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-service-worker-registration-interface">5.1. Extensions to the ServiceWorkerRegistration interface</a> <a href="#ref-for-service-worker-registration-interface①">(2)</a> <a href="#ref-for-service-worker-registration-interface②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-active-worker">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-active-worker">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-active-worker</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-active-worker">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-extend-lifetime-promises">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-extend-lifetime-promises">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-extend-lifetime-promises</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-extend-lifetime-promises">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-fire-functional-event">
   <a href="https://w3c.github.io/ServiceWorker/#fire-functional-event">https://w3c.github.io/ServiceWorker/#fire-functional-event</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-fire-functional-event">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-service-worker-client-frame-type">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client-frame-type">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client-frame-type</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-service-worker-client-frame-type">2. Concepts</a>
    <li><a href="#ref-for-dfn-service-worker-client-frame-type①">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-service-worker-concept">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-concept</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-service-worker-concept">1. Introduction</a> <a href="#ref-for-service-worker-concept①">(2)</a>
    <li><a href="#ref-for-service-worker-concept②">2. Concepts</a>
    <li><a href="#ref-for-service-worker-concept③">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-dfn-service-worker-client">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#dfn-service-worker-client</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dfn-service-worker-client">2. Concepts</a>
    <li><a href="#ref-for-dfn-service-worker-client①">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-service-worker-registration-concept">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-concept</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-service-worker-registration-concept">3. Constructs</a> <a href="#ref-for-service-worker-registration-concept①">(2)</a>
    <li><a href="#ref-for-service-worker-registration-concept②">5.1. Extensions to the ServiceWorkerRegistration interface</a>
    <li><a href="#ref-for-service-worker-registration-concept③">5.2. SyncManager interface</a> <a href="#ref-for-service-worker-registration-concept④">(2)</a> <a href="#ref-for-service-worker-registration-concept⑤">(3)</a>
    <li><a href="#ref-for-service-worker-registration-concept⑥">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-terminate-service-worker-algorithm">
   <a href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#terminate-service-worker-algorithm">https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#terminate-service-worker-algorithm</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-terminate-service-worker-algorithm">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-DOMString">
   <a href="https://heycam.github.io/webidl/#idl-DOMString">https://heycam.github.io/webidl/#idl-DOMString</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-DOMString">5.2. SyncManager interface</a> <a href="#ref-for-idl-DOMString①">(2)</a>
    <li><a href="#ref-for-idl-DOMString②">5.3. The sync event</a> <a href="#ref-for-idl-DOMString③">(2)</a> <a href="#ref-for-idl-DOMString④">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-Exposed">
   <a href="https://heycam.github.io/webidl/#Exposed">https://heycam.github.io/webidl/#Exposed</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-Exposed">5.2. SyncManager interface</a>
    <li><a href="#ref-for-Exposed①">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-invalidaccesserror">
   <a href="https://heycam.github.io/webidl/#invalidaccesserror">https://heycam.github.io/webidl/#invalidaccesserror</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-invalidaccesserror">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-invalidstateerror">
   <a href="https://heycam.github.io/webidl/#invalidstateerror">https://heycam.github.io/webidl/#invalidstateerror</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-invalidstateerror">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-notallowederror">
   <a href="https://heycam.github.io/webidl/#notallowederror">https://heycam.github.io/webidl/#notallowederror</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-notallowederror">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-boolean">
   <a href="https://heycam.github.io/webidl/#idl-boolean">https://heycam.github.io/webidl/#idl-boolean</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-boolean">5.3. The sync event</a> <a href="#ref-for-idl-boolean①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-idl-sequence">
   <a href="https://heycam.github.io/webidl/#idl-sequence">https://heycam.github.io/webidl/#idl-sequence</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-idl-sequence">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[ECMASCRIPT]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-sec-algorithm-conventions" style="color:initial">assert</span>
    </ul>
   <li>
    <a data-link-type="biblio">[HTML]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-eventhandler" style="color:initial">EventHandler</span>
     <li><span class="dfn-paneled" id="term-for-browsing-context" style="color:initial">browsing context</span>
     <li><span class="dfn-paneled" id="term-for-in-parallel" style="color:initial">in parallel</span>
    </ul>
   <li>
    <a data-link-type="biblio">[promises-guide]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-a-new-promise" style="color:initial">a new promise</span>
     <li><span class="dfn-paneled" id="term-for-reject-promise" style="color:initial">reject</span>
     <li><span class="dfn-paneled" id="term-for-resolve-promise" style="color:initial">resolve</span>
     <li><span class="dfn-paneled" id="term-for-upon-fulfillment" style="color:initial">upon fulfillment</span>
     <li><span class="dfn-paneled" id="term-for-upon-rejection" style="color:initial">upon rejection</span>
     <li><span class="dfn-paneled" id="term-for-waiting-for-all" style="color:initial">waiting for all</span>
    </ul>
   <li>
    <a data-link-type="biblio">[secure-contexts]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-secure-context" style="color:initial">secure context</span>
    </ul>
   <li>
    <a data-link-type="biblio">[service-workers-1]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-extendable-event-interface" style="color:initial">ExtendableEvent</span>
     <li><span class="dfn-paneled" id="term-for-extendable-event-init-dictionary" style="color:initial">ExtendableEventInit</span>
     <li><span class="dfn-paneled" id="term-for-service-worker-global-scope-interface" style="color:initial">ServiceWorkerGlobalScope</span>
     <li><span class="dfn-paneled" id="term-for-service-worker-registration-interface" style="color:initial">ServiceWorkerRegistration</span>
     <li><span class="dfn-paneled" id="term-for-dfn-active-worker" style="color:initial">active worker</span>
     <li><span class="dfn-paneled" id="term-for-dfn-extend-lifetime-promises" style="color:initial">extended lifetime promises</span>
     <li><span class="dfn-paneled" id="term-for-fire-functional-event" style="color:initial">fire functional event</span>
     <li><span class="dfn-paneled" id="term-for-dfn-service-worker-client-frame-type" style="color:initial">frame type</span>
     <li><span class="dfn-paneled" id="term-for-service-worker-concept" style="color:initial">service worker</span>
     <li><span class="dfn-paneled" id="term-for-dfn-service-worker-client" style="color:initial">service worker client</span>
     <li><span class="dfn-paneled" id="term-for-service-worker-registration-concept" style="color:initial">service worker registration</span>
     <li><span class="dfn-paneled" id="term-for-terminate-service-worker-algorithm" style="color:initial">termination</span>
    </ul>
   <li>
    <a data-link-type="biblio">[WebIDL]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-idl-DOMString" style="color:initial">DOMString</span>
     <li><span class="dfn-paneled" id="term-for-Exposed" style="color:initial">Exposed</span>
     <li><span class="dfn-paneled" id="term-for-invalidaccesserror" style="color:initial">InvalidAccessError</span>
     <li><span class="dfn-paneled" id="term-for-invalidstateerror" style="color:initial">InvalidStateError</span>
     <li><span class="dfn-paneled" id="term-for-notallowederror" style="color:initial">NotAllowedError</span>
     <li><span class="dfn-paneled" id="term-for-idl-boolean" style="color:initial">boolean</span>
     <li><span class="dfn-paneled" id="term-for-idl-sequence" style="color:initial">sequence</span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-ecmascript">[ECMASCRIPT]
   <dd><a href="https://tc39.github.io/ecma262/">ECMAScript Language Specification</a>. URL: <a href="https://tc39.github.io/ecma262/">https://tc39.github.io/ecma262/</a>
   <dt id="biblio-html">[HTML]
   <dd>Anne van Kesteren; et al. <a href="https://html.spec.whatwg.org/multipage/">HTML Standard</a>. Living Standard. URL: <a href="https://html.spec.whatwg.org/multipage/">https://html.spec.whatwg.org/multipage/</a>
   <dt id="biblio-promises-guide">[PROMISES-GUIDE]
   <dd><a href="https://www.w3.org/2001/tag/doc/promises-guide">Writing Promise-Using Specifications</a>. 24 July 2015. Finding of the W3C TAG. URL: <a href="https://www.w3.org/2001/tag/doc/promises-guide">https://www.w3.org/2001/tag/doc/promises-guide</a>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
   <dt id="biblio-secure-contexts">[SECURE-CONTEXTS]
   <dd>Mike West. <a href="https://www.w3.org/TR/secure-contexts/">Secure Contexts</a>. 15 September 2016. CR. URL: <a href="https://www.w3.org/TR/secure-contexts/">https://www.w3.org/TR/secure-contexts/</a>
   <dt id="biblio-service-workers-1">[SERVICE-WORKERS-1]
   <dd>Alex Russell; et al. <a href="https://www.w3.org/TR/service-workers-1/">Service Workers 1</a>. 2 November 2017. WD. URL: <a href="https://www.w3.org/TR/service-workers-1/">https://www.w3.org/TR/service-workers-1/</a>
   <dt id="biblio-webidl">[WebIDL]
   <dd>Boris Zbarsky. <a href="https://heycam.github.io/webidl/">Web IDL</a>. 15 December 2016. ED. URL: <a href="https://heycam.github.io/webidl/">https://heycam.github.io/webidl/</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="idl-index"><span class="content">IDL Index</span><a class="self-link" href="#idl-index"></a></h2>
<pre class="idl highlight def"><c- b>partial</c-> <c- b>interface</c-> <a class="idl-code" data-link-type="interface" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-registration-interface" id="ref-for-service-worker-registration-interface①①"><c- g>ServiceWorkerRegistration</c-></a> {
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="n" data-link-type="idl-name" href="#syncmanager" id="ref-for-syncmanager⑤"><c- n>SyncManager</c-></a> <a data-readonly data-type="SyncManager" href="#dom-serviceworkerregistration-sync"><code><c- g>sync</c-></code></a>;
};

[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed②"><c- g>Exposed</c-></a>=(<c- n>Window</c->,<c- n>Worker</c->)]
<c- b>interface</c-> <a href="#syncmanager"><code><c- g>SyncManager</c-></code></a> {
  <c- b>Promise</c->&lt;<c- b>void</c->> <a class="idl-code" data-link-type="method" href="#dom-syncmanager-register" id="ref-for-dom-syncmanager-register①"><c- g>register</c-></a>(<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString⑤"><c- b>DOMString</c-></a> <a href="#dom-syncmanager-register-tag-tag"><code><c- g>tag</c-></code></a>);
  <c- b>Promise</c->&lt;<c- b>sequence</c->&lt;<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString①①"><c- b>DOMString</c-></a>>> <a class="idl-code" data-link-type="method" href="#dom-syncmanager-gettags" id="ref-for-dom-syncmanager-gettags①"><c- g>getTags</c-></a>();
};

<c- b>partial</c-> <c- b>interface</c-> <a class="idl-code" data-link-type="interface" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#service-worker-global-scope-interface" id="ref-for-service-worker-global-scope-interface①"><c- g>ServiceWorkerGlobalScope</c-></a> {
  <c- b>attribute</c-> <a class="n" data-link-type="idl-name" href="https://html.spec.whatwg.org/multipage/webappapis.html#eventhandler" id="ref-for-eventhandler①"><c- n>EventHandler</c-></a> <a data-type="EventHandler" href="#dom-serviceworkerglobalscope-onsync"><code><c- g>onsync</c-></code></a>;
};

[<a class="idl-code" data-link-type="extended-attribute" href="https://heycam.github.io/webidl/#Exposed" id="ref-for-Exposed①①"><c- g>Exposed</c-></a>=<c- n>ServiceWorker</c->]
<c- b>interface</c-> <a href="#syncevent"><code><c- g>SyncEvent</c-></code></a> : <a class="n" data-link-type="idl-name" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-interface" id="ref-for-extendable-event-interface②"><c- n>ExtendableEvent</c-></a> {
  <a href="#dom-syncevent-syncevent"><code><c- g>constructor</c-></code></a>(<a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString②①"><c- b>DOMString</c-></a> <a href="#dom-syncevent-syncevent-type-init-type"><code><c- g>type</c-></code></a>, <a class="n" data-link-type="idl-name" href="#dictdef-synceventinit" id="ref-for-dictdef-synceventinit①"><c- n>SyncEventInit</c-></a> <a href="#dom-syncevent-syncevent-type-init-init"><code><c- g>init</c-></code></a>);
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString③①"><c- b>DOMString</c-></a> <a data-readonly data-type="DOMString" href="#dom-syncevent-tag"><code><c- g>tag</c-></code></a>;
  <c- b>readonly</c-> <c- b>attribute</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean②"><c- b>boolean</c-></a> <a data-readonly data-type="boolean" href="#dom-syncevent-lastchance"><code><c- g>lastChance</c-></code></a>;
};

<c- b>dictionary</c-> <a href="#dictdef-synceventinit"><code><c- g>SyncEventInit</c-></code></a> : <a class="n" data-link-type="idl-name" href="https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#extendable-event-init-dictionary" id="ref-for-extendable-event-init-dictionary①"><c- n>ExtendableEventInit</c-></a> {
  <c- b>required</c-> <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-DOMString" id="ref-for-idl-DOMString④①"><c- b>DOMString</c-></a> <a data-type="DOMString " href="#dom-synceventinit-tag"><code><c- g>tag</c-></code></a>;
  <a class="idl-code" data-link-type="interface" href="https://heycam.github.io/webidl/#idl-boolean" id="ref-for-idl-boolean①①"><c- b>boolean</c-></a> <a data-default="false" data-type="boolean " href="#dom-synceventinit-lastchance"><code><c- g>lastChance</c-></code></a> = <c- b>false</c->;
};

</pre>
  <aside class="dfn-panel" data-for="in-the-background">
   <b><a href="#in-the-background">#in-the-background</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-in-the-background">1. Introduction</a>
    <li><a href="#ref-for-in-the-background①">4.2. Location Tracking</a>
    <li><a href="#ref-for-in-the-background②">4.3. History Leaking</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="online">
   <b><a href="#online">#online</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-online">2. Concepts</a>
    <li><a href="#ref-for-online①">5.2. SyncManager interface</a> <a href="#ref-for-online②">(2)</a>
    <li><a href="#ref-for-online③">5.3. The sync event</a> <a href="#ref-for-online④">(2)</a> <a href="#ref-for-online⑤">(3)</a> <a href="#ref-for-online⑥">(4)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="list-of-sync-registrations">
   <b><a href="#list-of-sync-registrations">#list-of-sync-registrations</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-list-of-sync-registrations">3. Constructs</a>
    <li><a href="#ref-for-list-of-sync-registrations①">5.2. SyncManager interface</a> <a href="#ref-for-list-of-sync-registrations②">(2)</a> <a href="#ref-for-list-of-sync-registrations③">(3)</a>
    <li><a href="#ref-for-list-of-sync-registrations④">5.3. The sync event</a> <a href="#ref-for-list-of-sync-registrations⑤">(2)</a> <a href="#ref-for-list-of-sync-registrations⑥">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="sync-registration">
   <b><a href="#sync-registration">#sync-registration</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-sync-registration">2. Concepts</a>
    <li><a href="#ref-for-sync-registration①">3. Constructs</a> <a href="#ref-for-sync-registration②">(2)</a> <a href="#ref-for-sync-registration③">(3)</a> <a href="#ref-for-sync-registration④">(4)</a> <a href="#ref-for-sync-registration⑤">(5)</a>
    <li><a href="#ref-for-sync-registration⑥">5.2. SyncManager interface</a> <a href="#ref-for-sync-registration⑦">(2)</a>
    <li><a href="#ref-for-sync-registration⑧">5.3. The sync event</a> <a href="#ref-for-sync-registration⑨">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="tag">
   <b><a href="#tag">#tag</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-tag">3. Constructs</a> <a href="#ref-for-tag①">(2)</a>
    <li><a href="#ref-for-tag②">5.2. SyncManager interface</a> <a href="#ref-for-tag③">(2)</a> <a href="#ref-for-tag④">(3)</a>
    <li><a href="#ref-for-tag⑤">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="registration-state">
   <b><a href="#registration-state">#registration-state</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-registration-state">3. Constructs</a>
    <li><a href="#ref-for-registration-state①">5.2. SyncManager interface</a> <a href="#ref-for-registration-state②">(2)</a> <a href="#ref-for-registration-state③">(3)</a> <a href="#ref-for-registration-state④">(4)</a> <a href="#ref-for-registration-state⑤">(5)</a>
    <li><a href="#ref-for-registration-state⑥">5.3. The sync event</a> <a href="#ref-for-registration-state⑦">(2)</a> <a href="#ref-for-registration-state⑧">(3)</a> <a href="#ref-for-registration-state⑨">(4)</a> <a href="#ref-for-registration-state①⓪">(5)</a> <a href="#ref-for-registration-state①①">(6)</a> <a href="#ref-for-registration-state①②">(7)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="pending">
   <b><a href="#pending">#pending</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-pending">3. Constructs</a>
    <li><a href="#ref-for-pending①">5.2. SyncManager interface</a> <a href="#ref-for-pending②">(2)</a>
    <li><a href="#ref-for-pending③">5.3. The sync event</a> <a href="#ref-for-pending④">(2)</a> <a href="#ref-for-pending⑤">(3)</a> <a href="#ref-for-pending⑥">(4)</a> <a href="#ref-for-pending⑦">(5)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="waiting">
   <b><a href="#waiting">#waiting</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-waiting">5.2. SyncManager interface</a>
    <li><a href="#ref-for-waiting①">5.3. The sync event</a> <a href="#ref-for-waiting②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="firing">
   <b><a href="#firing">#firing</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-firing">5.2. SyncManager interface</a>
    <li><a href="#ref-for-firing①">5.3. The sync event</a> <a href="#ref-for-firing②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="reregisteredwhilefiring">
   <b><a href="#reregisteredwhilefiring">#reregisteredwhilefiring</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-reregisteredwhilefiring">5.2. SyncManager interface</a>
    <li><a href="#ref-for-reregisteredwhilefiring①">5.3. The sync event</a> <a href="#ref-for-reregisteredwhilefiring②">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="syncmanager">
   <b><a href="#syncmanager">#syncmanager</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-syncmanager">5.1. Extensions to the ServiceWorkerRegistration interface</a> <a href="#ref-for-syncmanager①">(2)</a>
    <li><a href="#ref-for-syncmanager②">5.2. SyncManager interface</a> <a href="#ref-for-syncmanager③">(2)</a> <a href="#ref-for-syncmanager④">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-syncmanager-register">
   <b><a href="#dom-syncmanager-register">#dom-syncmanager-register</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-syncmanager-register">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-syncmanager-gettags">
   <b><a href="#dom-syncmanager-gettags">#dom-syncmanager-gettags</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-syncmanager-gettags">5.2. SyncManager interface</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="syncevent">
   <b><a href="#syncevent">#syncevent</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-syncevent">5.3. The sync event</a> <a href="#ref-for-syncevent①">(2)</a> <a href="#ref-for-syncevent②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-syncevent-tag">
   <b><a href="#dom-syncevent-tag">#dom-syncevent-tag</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-syncevent-tag">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dom-syncevent-lastchance">
   <b><a href="#dom-syncevent-lastchance">#dom-syncevent-lastchance</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dom-syncevent-lastchance">5.3. The sync event</a> <a href="#ref-for-dom-syncevent-lastchance①">(2)</a> <a href="#ref-for-dom-syncevent-lastchance②">(3)</a> <a href="#ref-for-dom-syncevent-lastchance③">(4)</a> <a href="#ref-for-dom-syncevent-lastchance④">(5)</a> <a href="#ref-for-dom-syncevent-lastchance⑤">(6)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="dictdef-synceventinit">
   <b><a href="#dictdef-synceventinit">#dictdef-synceventinit</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-dictdef-synceventinit">5.3. The sync event</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="fire-a-sync-event">
   <b><a href="#fire-a-sync-event">#fire-a-sync-event</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-fire-a-sync-event">5.2. SyncManager interface</a> <a href="#ref-for-fire-a-sync-event①">(2)</a>
    <li><a href="#ref-for-fire-a-sync-event②">5.3. The sync event</a> <a href="#ref-for-fire-a-sync-event③">(2)</a> <a href="#ref-for-fire-a-sync-event④">(3)</a> <a href="#ref-for-fire-a-sync-event⑤">(4)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="will-retry">
   <b><a href="#will-retry">#will-retry</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-will-retry">5.3. The sync event</a>
   </ul>
  </aside>
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